Open source MahTweets is a solid, extensible Twitter client for Windows
If you're willing to overlook the...er...interesting interface, MahTweets is loaded with features. It's built on .Net 3.5 and the Windows Presentation Foundation and is available as a ClickOnce installer. CPU and memory usage are very respectable, and updates are sent and received rapidly.
Twitter power users won't like the fact that MahTweets doesn't support search columns. More casual, conversational users probably won't notice the omission. The app does provide a good way to view the big three - everything, direct messages, and @ mentions.
[via Doug Finke]
Out of the box, MahTweets supports uploading images to Flickr, YFrog, TwitPic, Twitgoo, and Thumbwhere. Webcam capture support is also built in.
URLs can be automatically shortened with either is.gd or tinyurl. Nine color scheme presets are provided and blue, black, and white all look decent.
Facebook is also supported, though it's still somewhat lean. Scrolling through your stream feels a bit clunky, and comments don't appear after you submit them. My timeline also appears to be less-than-current, only showing items more than 12 hours old. Not quite the realtime feed MahTweets pulls from Twitter.
As far as straight-up, three-column Twitter clients for Windows go, though, MahTweets is a solid option.